“Why is no temperature displayed for my external FireWire or USB hard disk?

Nearly all FireWire or USB hard disks are in fact ATA hard disks accessed via a “FireWire/USB to ATA bridge device”. Those bridge chips only translate a subset of the ATA commands between the FireWire/USB and the ATA protocol standards. In particular, commands to transfer S.M.A.R.T. diagnostic data (which are needed to access the temperature sensors inside the drives) are usually not supported. For this reason it is technically impossible to transport temperature data across a bridge. There will be a future industry standard called “Smart Command Transfer (SCT)” which will address this issue. But it will take some time before Apple and the hard disk vendors can fully support this.

Use the menu item Window > Show Drive Overview or Press Apple+8 to display the transport protocols of all disk drives and their S.M.A.R.T. capabilities.”